In telephone switching equipment, communication equipment, computer equipment, and in many other applications, the need for an uninterrupted source of DC power is critical. Rectified commercial AC power is typically used as the primary source of DC power for such equipment.
To avoid any interruption or outage in power service, it is common practice to employ a battery back-up for the primary DC source. Back-up battery systems typically include strings of batteries or cells connected in parallel with the primary DC source and the load. In the event of a drop in the load bus voltage below a predetermined threshold, the back-up battery supplants or supplements the primary source of DC power. Back-up battery systems are designed to replace the primary DC power source for a predetermined period of time within which resumption of primary power is expected to occur.
In conventional back-up battery systems, the nominal system load bus voltage has typically been dictated by battery characteristics. For example, in a telephone switching plant, back-up batteries are commonly employed which have a design cell voltage of -2.26 volts, for optimum health of the battery cell. Twenty-four cells are typically combined in a string, resulting in a nominal load bus voltage of approximately -54 volts. A bank of strings supplies the necessary back-up DC power.
As the back-up batteries are placed across the load, the full 54 volts of system DC voltage are placed across the battery string. This design architecture of a typical back-up battery system presents a number of potential problems. Certain batteries, due to their electrochemical constitution, will draw more current than other batteries. All batteries, as they age, will experience increasing internal resistance and will draw more charging current from the main DC supply.
About a decade ago, a new type of lead acid battery was introduced into the marketplace. The battery is sealed, and allegedly requires no maintenance. In this type of battery, oxygen and hydrogen produced during electrochemical reactions in the battery recombine to maintain an aqueous liquid electrolyte at a constant level within the cell. As a result, these batteries have only a small amount of liquid electrolyte. These batteries have become known as "valve regulated", or "recombinant"or "electrolyte-starved" batteries.
This type of lead acid battery (hereinafter termed "valve regulated lead acid " or "VRLA" batteries) has often failed well before their design life, which is typically 10 years.
A particular battery may, for various reasons not clearly understood, begin to take on more amperage to maintain its charge. The increasing charging current will elevate the temperature of the battery. The chemical recombination of the oxygen and hydrogen gases also creates heat. As the internal battery temperature increases, the current demand increases disproportionately. For every 10 degrees centigrade of increase in the battery's internal temperature, the current demand doubles. A battery in this condition will have one of two failure modes, the most damaging being "thermal runaway." Thermal runaway may lead to an explosion of the battery, with likely destruction or severe damage to any nearby equipment. Alternatively, the battery may experience a "melt down" and produce noxious gases which are also apt to damage or destroy neighboring equipment.
The rectified AC source provided in typical telephone switching plants has more than ample capacity to supply any one or more batteries demanding abnormal charging current, thus encouraging the aforedescribed thermal runaway or meltdown failures.
With the advent of fiber optic signal distribution, switching equipment has been decentralized, introducing a need for DC power supplies in unattended satellite installations distributed throughout the territory served. In these unattended installations, the equipment is often closely packed, leading to hostile thermal operating conditions for the equipment and increased occurrences of thermally induced failures.
In less severe conditions, the placement of the back-up batteries directly across the load is apt to result in dry-out (loss of electrolyte), positive grid corrosion, and other problems which may lead to premature battery failure and/or sub-normal power performance.
Back-up battery systems must be monitored to determine the health and capacity of the batteries. The need to perform battery tests is particularly troublesome in systems which require the supply of an uninterrupted source of DC power. Testing of the vital statistics of a battery affecting output capacity, predicted life, etc. is presently done by taking the battery strings off-line and testing them in one of two ways. The test procedure recommended by battery manufacturers as being the most reliable is to discharge the battery into a load while measuring the response of the battery. The ability of a battery or battery string to hold a predetermined current level for a predetermined time is a reliable measure of the health and capacity of the battery. However, such discharge tests in the field require experienced personnel and are difficult and costly. Further, conventional battery testing, requiring the batteries to be taken offline, suffers a loss of standby battery protection for the telephone plant or other equipment being supplied while the tested batteries are off-line.
To avoid the cost and inconvenience of a discharge test, it is commonplace to employ special field test equipment which tests for battery resistance, impedance, inductance, and other parameters and characteristics without discharging the battery. See U.S. Pat. No. 5,250,904. However, as noted, tests which do not involve discharging the battery are apt to be less reliable.
U.S. Pat. No. 5,160,851 to Joseph M. McAndrews, one of the present inventors, discloses a back-up battery system for telephone central office switching equipment. The back-up battery system includes one or more rechargeable batteries having cells floated at a given float voltage. The cells are of a number such that when the batteries are switched in circuit across the load, the cumulative voltage of the batteries exceeds a predetermined load voltage for a preselected period. The over-voltage that results from the switching in of extra cells across the load is down converted by a converter. The converter, a sensor for sensing the system discharge bus voltage, and a switch may be formed as a single unit using MOSFET technology. It is said that in such case a fail-safe contact switch might also be provided to parallel the MOSFET switch and be operated in the event of its failure.